US President Obama will travel to Germany next week, but his visit is not likely to be all smiles and rainbows when the conversation turns to PRISM. Leutheusser-Schnarrenberger noted that the US president effectively said that there is no alternative to a data surveillance system in the modern age, and she disputed this.

"On the weekend, President Obama reacted by saying that it is impossible to have 100 percent security and 100 percent privacy and zero inconvenience," she wrote.

"I don't share this view. The more a society monitors, controls and observes its citizens, the less free it is. In a democratic constitutional state, security is not an end in itself, but serves to secure freedom."

Leutheusser-Schnarrenberger further implied that US surveillance of German citizens is not acceptable, and she called for the US government to be more open and forthcoming.

"The suspicion of excessive surveillance of communication is so alarming that it cannot be ignored. For that reason, openness and clarification by the US administration itself should be paramount at this point. All facts must be put on the table," she added.

"The global internet has become indispensible for a competitive economy, the sharing of information and the strengthening of human rights in authoritarian countries. But our trust in these technologies threatens to be lost in the face of comprehensive surveillance activities."

Meanwhile, European Union (EU) Justice Commissioner Viviane Reding has written to the US attorney general and demanded answers.

I have asked the US Attorney-General for detailed explanations about #PRISM

She confirmed the letter in a stream of anti-PRISM messages on Twitter, messages that accused the US government of overreach and US companies of colluding with it.

"The respect for fundamental rights and the rule of law are the foundations of the EU-US relationship," she said. "The rights of EU citizens can be but undermined by US companies which give priority to US laws over #EU fundamental rights." µ